Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

How many EDTA (ethylenediamine-tetraacetic acid) molecules are required to make an octahedral complex with a \(\mathrm{Ca}^{2+}\) ion? (a) six (b) three (c) one (d) two

Step-by-Step Solution

Answer

One EDTA molecule is required to make an octahedral complex with a \( \mathrm{Ca}^{2+} \) ion.

Step 1: Understanding Chelation

EDTA is a hexadentate ligand, meaning it can form six bonds to a central metal ion, like \( \mathrm{Ca}^{2+} \). This property allows it to fully occupy all coordination sites of metals in an octahedral complex.

Step 2: Coordination Number of Calcium

Calcium \( (\mathrm{Ca}^{2+}) \) ions typically form complexes with a coordination number of six, meaning they can bond with six atoms or groups.

Step 3: Forming an Octahedral Complex

The ability of EDTA to use its six donor atoms to bind to a \( \mathrm{Ca}^{2+} \) ion perfectly fills the coordination number. Thus, only one EDTA molecule is required to form an octahedral complex with a \( \mathrm{Ca}^{2+} \) ion.

Key Concepts

Octahedral Complexes

An octahedral complex is a type of coordination entity where a central metal ion is surrounded by six ligands. This arrangement forms a shape similar to two square pyramids joined at their bases.
These types of complexes are commonly seen in coordination chemistry due to their symmetrical and stable structure. Octahedral complexes have various distinctive features:

• They consist of central metal ions that are typically transition metals, but can also include other elements like lanthanides or actinides.
• Ligands in octahedral complexes can be either neutral molecules or ions that possess lone pairs of electrons. These electrons are donated to the metal ion to form coordinate covalent bonds.
• The formation of an octahedral complex can significantly affect the properties of the metal ion, including its magnetic and electronic properties.

In the case of the EDTA and calcium ion, the complex is octahedral because the calcium ion is coordinated to six donor atoms from a single EDTA molecule.

Hexadentate Ligand

A hexadentate ligand, such as EDTA (ethylenediamine-tetraacetic acid), is a molecule that is capable of forming six bonds with a central metal ion. The term **'hexadentate'** is derived from 'hexa' meaning six and 'dentate' meaning 'teeth,' symbolizing the ligand's ability to grip the metal ion at multiple points.
This is significant as hexadentate ligands can effectively envelop the metal ion, making them excellent for creating stable chelate complexes. Key characteristics of hexadentate ligands include:

• High stability: The multiple bonds formed by the hexadentate ligand create strong and stable chelate complexes.
• Versatility: Although typically involved in forming complexes with metal ions, hexadentate ligands can bind to a variety of different ions.
• Complete coordination: They often fill all coordination sites of the metal ion, particularly in octahedral complexes.

In the context of EDTA and calcium ions, EDTA acts as a hexadentate ligand, using its six donor atoms to fully coordinate and stabilize the octahedral complex.

Coordination Number

The coordination number of a metal ion is the number of atoms, ions, or molecules that can surround the central metal atom or ion in a complex. It indicates how many connections the metal ion can make with ligands.
Coordination numbers are essential in defining the geometry and overall structure of the complex. Consider these points when thinking about coordination numbers:

• Common coordination numbers range from 2 to 12, but 4 and 6 are the most prevalent, especially in octahedral and tetrahedral geometries.
• The coordination number is determined by the size, charge, and electronic configuration of the metal ion, as well as the size and binding ability of the ligands.
• Higher coordination numbers suggest more crowded coordination environments, which can affect the chemistry and reactivity of the complex.

In the case of the calcium ion in our exercise, its coordination number is 6, which matches the six donor sites of the EDTA molecule, leading to the formation of a stable octahedral complex via complete coordination.